1. Field of the Invention
The present invention relates to a drive unit, and more particularly to a drive unit which is preferable for a lens drive mechanism or the like in an optical equipment, for example, an imaging apparatus (a digital camera, a video camera and the like), an optical pickup apparatus, an optical measuring equipment or the like.
2. Description of the Related Art
As a drive unit applied to a lens drive mechanism in an imaging apparatus, an optical pickup apparatus or the like, various types of structures have been conventionally proposed. However, classifying roughly while paying attention to a drive source thereof, there are a magnetic force source type employing an electromagnetic motor or the like as a drive source, and a non-magnetic force source type employing a piezoelectric actuator or the like as the drive source. As an example of the former, for example, in Japanese Unexamined Patent Publication No. 8-275496, there is disclosed a drive unit provided with a propelling field magnet which is fixed to a movable member so as to be relatively at a standstill with respect to the movable member.
In this drive unit, the structure is made such that a position of the movable member is detected on the basis of a displacement amount detected by a magnetic sensor which is integrally provided with the movable member, however, since the drive source displacing the movable member is constituted by the electromagnetic motor, a high pass filter treatment is applied in correspondence to a drive speed of the movable member so as to remove an offset in such a manner that no offset is superposed on an output of the magnetic sensor by a leakage flux generated from the field magnet.
On the other hand, as an example of the latter, there are listed up a drive unit described in Japanese Unexamined Patent Publication No. 2000-205809, Japanese Unexamined Patent Publication No. 2003-185406 and Japanese Unexamined Patent Publication No. 2004-39068. For example, in Japanese Unexamined Patent Publication No. 2000-205809, a drive unit having a piezoelectric actuator as the drive source is disclosed. In the drive unit, there is employed a position detecting method of the movable member of detecting a position of the movable member which is frictionally engaged with the drive member by utilizing an electric resistance of the drive member fixed to one end of a piezoelectric element. Japanese Unexamined Patent Publication No. 2003-185406 discloses a drive unit having a piezoelectric actuator as the drive source, and discloses a method of detecting the position of the movable member on the basis of an electrostatic capacity change between a movable electrode provided in the movable member and a fixed electrode provided in a fixed portion. Further, Japanese Unexamined Patent Publication No. 2004-39068 discloses a drive unit moving a lens by using a piezoelectric actuator as the drive source for compensating a spherical aberration caused by a thickness unevenness of a cover layer of an optical disc, however, a structure relating to the position detection of the lens is not shown.
However, in the drive unit described in Japanese Unexamined Patent Publication No. 8-275496, it is necessary to apply the high pass filter treatment against the problem that the offset is superposed on the output of the magnetic sensor by the leakage flux generated from the drive source (the electromagnetic motor). Accordingly, a detection circuit becomes complicated, and the structure becomes disadvantageous in view of a cost and a reliability. Further, there is a problem that it is hard to manufacture a field magnet in which N poles and S poles are alternately arranged, precisely and with a high resolution.
On the other hand, in the drive units described in Japanese Unexamined Patent Publication No. 2000-205809 and Japanese Unexamined Patent Publication No. 2003-185406, the problem mentioned above is not generated, however, the following disadvantage is generated in a point of detecting the position of the movable member. In other words, in the drive unit described in Japanese Unexamined Patent Publication No. 2000-205809, since a contact type sensing method of detecting the position of the movable member by utilizing an electric resistance of the drive member, a contact resistance between the movable member and the drive member fluctuates, and it is hard to obtain a high resolution. In order to improve an actuator performance, the drive member having a reduced weight and a high rigidity is desired, however, there is a problem that it is hard to select a material of the drive member for establishing both of the electric resistance value and the high rigidity for sensing. The drive unit described in Japanese Unexamined Patent Publication No. 2003-185406 employs a non-contact type sensing means, however, since it is necessary to apply an alternating voltage to the movable electrode or the fixed electrode, the detection circuit becomes complicated and the structure becomes disadvantageous in view of the cost and the reliability. Further, there is a problem that it is necessary to make a gap between the fixed electrode and the movable electrode extremely small for obtaining a high resolution.
Further, as a problem from the other view point, there can be listed up a reduction of the position detecting precision caused by the fluctuation of an operating environment of the drive unit. For example, the method in Japanese Unexamined Patent Publication No. 8-275496 employs the magnetic sensor for detecting the position of the movable member, however, since a sensing property thereof displaces on the basis of an environmental temperature or the like, the position detecting precision of the movable member is lowered by the change of the environmental temperature or the like. Further, in the case that a plurality of position detecting points of the movable member exist at a wide interval, a necessary position precision can not be obtained, and each of the members and the space are increased in accordance with an increase in a moving amount of the movable member.